This invention relates to a shield of electromagnetic interference (EMI). More particularly, the invention relates to an EMI shield comprising a nonconductive substrate overlaid with an undercoat which, in turn, is overlaid with a metal layer. The invention also relates to paint compositions for applying said undercoat.
With the recent development of an advanced information-intensive society, the use of electric waves over various frequency bands is rapidly increasing in such areas as multi-channel access systems and mobile communication equipment typified by cellular phones. This trend will be accelerated if the PHS (personal handy-phone system) and radio-LAN which has recently been put to field use becomes commonplace. Personal computers and other electronic equipment for consumer use that incorporate microcomputers have already become popular tools. As one can easily imagine, the electric waves radiated from these apparatus will add to the already versatile and complicated environment associated with electric waves; in addition, the use of higher-frequency bands will increase in the years to come.
In this situation, the electromagnetic interference and/or radio frequency interference resulting from various electronic equipment has been a cause of various troubles such as erroneous operations of other electronic equipment, electronic parts, computers and machining tools, leakage of information, and noise to television sets and radio receivers. The occurrence of such troubles has been increasing every year and a further increase is predicted for the future.
If electronic equipment and parts thereof are jacketed or encased with plastics, they are practically transparent to disturbing electromagnetic waves and/or radio frequency interference unless appropriate protections against EMI are applied.
Means commonly employed for this purpose are shields of electromagnetic interference and electric wave absorbing materials. The former is such that electromagnetic waves coming from outside are reflected and thereby prevented from propagation into the electronic apparatus or part thereof to be protected or, alternatively, electromagnetic waves coming from inside are reflected such that they will not radiate out of the electronic apparatus or part thereof. Electric wave absorbing materials are such that the incident electric waves are converted to thermal energy so that the intensity of their transmission or reflection is reduced markedly.
Ferrites and carbon are extensively used as electric wave absorbing materials. Sintered ferrites are effective at comparatively low frequencies such as those in the VHF band whereas carbon is effective at comparatively high frequencies. Both ferrites and carbon can be used as mixtures, or dispersions in organic substances such as rubbers or plastics; in this case, the absorption characteristics can be controlled by adjusting the content of the electric wave absorber or by using more than one electric wave absorber. Carbon-based electric wave absorbers are frequently used as such mixtures and those which incorporate carbon black and/or graphite particles have been commercialized.
However, the prior art carbon-based electric wave absorbers incorporating carbon black and/or graphite particles are not considered to exhibit satisfactory absorbing characteristics and a further improvement in performance has been desired. As already mentioned, ferrites are effective in absorbing electromagnetic waves at frequencies of about several hundred MHz but are not capable of satisfactory absorption of electromagnetic waves at higher frequencies in the GHz band which is anticipated to find increased use in the future. In contrast, carbon black is capable of absorbing electromagnetic waves at frequencies in the GHz band but its absorption zone is not very wide.
Other materials that are effective as absorbers of electric waves include aluminum, lead, zinc, titanium, lithium, stainless steel, silver, copper and fibers. In particular, PZT (lead zirconate titanate) and PLZT (lead lanthanum zirconate titanate) are effective in absorbing electric waves at frequencies in the GHz band; however, the amount by which these materials can be dispersed or incorporated in plastics is limited by various factors including the melt viscosity of the plastics used, their processability, the mechanical strength, brittleness and adhesion of fibers, films, sheets and other shaped parts of the plastics in which those metals are dispersed or incorporated. It is also known that if the incorporation of conductive metals is unduly small, satisfactory absorption characteristics are not attained.
Under the circumstances, electronic equipment and parts thereof jacketed or encased with plastics are commonly protected against disturbing electromagnetic waves and/or radio frequency interference by means of EMI shields rather than electric wave absorbers. Methods currently employed to form EMI shields over suitable surfaces of plastic jackets or cases include the application of conductive paints, metal-arc spraying, vacuum metallizing, evaporating, cladding, and deposition of metal layers (by electroplating or electroless plating).
In practice, however, it is not always easy to form EMI shields over surfaces of plastic jackets or cases and the plastic surfaces have to be given a preliminary treatment in order to ensure positive depositing of the intended EMI shields. However, mechanical pretreatments such as surface roughening may occasionally destroy the plastic surfaces per se.
To avoid this problem, primer paints are applied to form undercoats which, in turn, are overlaid with EMI shields. However, the heretofore used primer paints mostly contain metal particles as a film-forming component and, as is well known, such metal particles will react with the water in the atmosphere to form metal oxides which, in turn, cause the applied primer coat to swell, eventually leading to delamination. Alternatively, the organic solvents used in the primer paints will cause delamination of the applied primer coat (see, for example, Japanese Domestic Announcement (kohyo) No. 50034/1987). Another potential cause of delamination is the crazing which may occur during shaping. For whichever reason, the EMI shields formed over the undercoat will be destroyed to lose the ability to protect electronic equipment or parts thereof against disturbing electromagnetic waves and/or radio frequency interference.